(Invited) Closed Bipolar Electrochemistry for Sensor and Bioimaging Applications

Closed bipolar electrochemistry (cBPE) involves simultaneous oxidation and reduction at the opposite poles of a conductor located in two separated solutions. This unique property is expected to be used as new electrochemical analytical tools including sensors and imaging platforms. I will introduce our researches for application of cBPE to sensors and bioimaging. Our first application of cBPE is in a sensor device for liquid-junction-free substitutional stripping voltammetry (SSV) (Figure A) [1]. SSV is a method for highly sensitive amperometric measurement by integrating small oxidation current over time as a silver deposition. A general SSV uses a galvanic-like cell with a liquid junction to convert an oxidation current generated in one cell into silver deposition in the other cell. Then, the amount of silver deposition is quantified by stripping voltammetry. As an idea of eliminating the liquid junction prohibiting the miniaturization of the SSV device, we proposed a method using conductor alternative to liquid junction to obtain electrical continuity between two cells. I later realized that this configuration is the cBPE. We also applied cBPE to unique amperometric sensors including simultaneous multiple measurement device (Figure B) [2] and probe-type device integrating a reference electrode [3]. Our second application of cBPE is in an electrochemical imaging platform. In order to realize an electrochemical microscope for real-time bioimaging with single-cell level spacial resolution, we have employed closed bipolar electrode array with electrochemiluminescence (ECL) detecting system (Figure C). As a first demonstration, we successfully obtained an image of flowing [Fe(CN)6]3- ions on a cBPE array fabricated by bundling Au wires (200 µm in diameter) at a frame rate of 10 fps [4]. Then, a higher density cBPE array of which interval was 35 µm pitch on average was fabricated via electroless plating on a micropore membrane without any photolithographic technique. Using this cBPE array, difference in respiratory activity of a live and a dead MCF-7 spheroids was successfully imaged as oxygen distribution on the cBPE array. Now we are on a research of operation of liquid junction free sensor using cBPE as well as researches for expanding the application of the cBPE based imaging platform. These studies are expected to further expand the possibilities of bipolar electrochemistry. References: [1] Shinichiro Takano et al., Electrochem. Commun., 66 (2016) 34-37. [2] Kumi Y. Inoue, et al., ChemElectroChem, 5 (2018) 2167-2170. [3] Siti Masturah binti Fakhruddin, et al., Electrochem. Commun., 93 (2018) 62-65. [4] Tomoki Iwama, et al., Chem. Lett., 47 (2018) 843-845. Figure 1